In the field of process measurements and automation technology, for measuring physical parameters, such as e.g. the mass flow, density and/or viscosity of media flowing in pipelines, often such measuring systems formed as inline measuring devices of compact construction are used, which, by means of a measuring transducer of vibration-type through which the medium flows, and a transmitter electronics connected thereto, effect reaction forces in the medium, such as e.g. Coriolis forces corresponding with the mass flow, inertial forces corresponding with the density of the medium, and/or frictional forces corresponding with the viscosity of the medium, and, derived from these, produce a measurement signal representing the respective mass flow, density and/or viscosity of the medium. Such measuring transducers, in part embodied also as multivariable Coriolis mass flow/viscosity meters or Coriolis mass flow/density/viscometer, are described in detail in e.g. EP-A 1 001 254, EP-A 553 939, U.S. Pat. No. 4,793,191, US-A 2002/0157479, US-A 2006/0150750, US-A 2007/0151368, US-A 2010/0050783, U.S. Pat. No. 5,370,002, U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,796,011, U.S. Pat. No. 6,308,580, U.S. Pat. No. 6,415,668, U.S. Pat. No. 6,711,958, U.S. Pat. No. 6,920,798, U.S. Pat. No. 7,134,347, U.S. Pat. No. 7,392,709, WO-A 96/08697, WO-A 03/027616, WO-A 2008/059262, WO-A 2009/120222 or WO-A 2009/120223.
Each of the measuring transducers includes a transducer housing, which is formed from an inlet-side, first housing end, at least partially by means of two or four, first flow divider, having in each case circularly cylindrical or conical flow openings spaced apart from one another, and from an outlet-side, second housing end formed at least partially by means of two or four, second flow divider, having in each case flow openings spaced apart from one another. In the case of at least some of the measuring transducers illustrated in U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,796,011, U.S. Pat. No. 7,350,421, or US-A 2007/0151368, the transducer housing comprises a rather thick walled, circularly cylindrical tube segment, which forms at least a middle segment of the transducer housing.
For conveying the at least sometimes flowing medium, the measuring transducers comprise furthermore, in each case, at least two measuring tubes connected for parallel flow—in each case straight, or in each case equally curved—made of metal, especially steel or titanium, which tubes are placed within the transducer housing, and are oscillatably held therein by means of the aforementioned flow dividers. A first of the equally constructed measuring tubes, extending parallel to the other, opens into a first flow opening of the inlet-side, first flow divider with an inlet-side, first measuring tube end, and into a first flow opening of the outlet-side, second flow divider with an outlet-side, second measuring tube end. A second of the measuring tubes opens into in a second flow opening of the first flow divider with an inlet-side, first measuring tube end, and into a second flow opening of the second flow divider with an outlet-side, second measuring tube end. Each of the flow dividers includes additionally, in each case, a flange with a sealing surface for the fluid-tight connecting of the measuring transducer to pipe segments of the pipeline serving to supply the medium to, or to carry the medium away from, the measuring transducer.
The measuring tubes of known measuring systems of the aforementioned type are caused to vibrate during operation for the purpose of producing the aforementioned reaction forces, driven in the so-called driven, or wanted, mode by an exciter mechanism serving to produce or maintain mechanical oscillations of the measuring tubes—in this case, bending oscillations about an imaginary oscillation axis, which imaginarily connects the respective first and second measuring tube ends. The oscillations in the wanted mode are, particularly also in applications of the measuring transducer in measuring systems formed as Coriolis mass flow- and/or density measuring devices, developed as lateral bending oscillations, and bear superimposed thereon, in the case of medium flowing through the measuring tubes, as a result of Coriolis forces induced therein, additional, equal frequency oscillations in the so-called Coriolis mode. Accordingly, the exciter mechanism—here most often electrodynamic—in the case of straight measuring tubes, is embodied in such a manner that the two measuring tubes in the wanted mode at least partially—most often, however, predominantly—can be excited differentially to opposite phase bending oscillations in a shared plane of oscillation; that is, by entry of exciter forces simultaneously along a shared line of action, however, acting in opposite directions by means of at least one oscillation exciter linked just to the two measuring tubes. As, among other things, evident from the mentioned US-A 2006/0150750, based on opposite phase bending oscillations of two measuring tubes, besides mass flow and density, the viscosity of the medium conveyed in the measuring transducer can also be ascertained, for instance, based on an electrical excitation power, fed from the transmitter electronics to the exciter mechanism, serving to overcome the damping of the measuring tube oscillations caused also particularly by the medium located in the measuring tubes.
For registering of vibrations, especially of oscillations of the measuring tubes excited by the exciter mechanism, and for producing oscillation measurement signals serving as vibration representing, primary signals of the measuring transducer, the measuring transducers have additionally, in each case, a sensor arrangement, most often likewise electrodynamic, which reacts to relative movements of the measuring tubes. Typically, the sensor arrangement is formed by means of an inlet-side oscillation sensor, registering oscillations of the measuring tubes differentially—thus only relative movements of the measuring tubes as well as of an outlet-side oscillation sensor, also registering oscillations of the measuring tubes differentially. Each of the normally equally constructed oscillation sensors is formed by means of a permanent magnet held on the first measuring tube, and a cylindrical coil, permeated by the magnetic field of the permanent magnet, held on the second measuring tube.
In operation, the above described tube arrangement, formed by means of the at least two measuring tubes, with the, in each case shared holding of the exciter mechanism and the sensor arrangement of the measuring transducer, is excited by means of the electromechanical exciter mechanism, at least at times, in the wanted mode, to execute mechanical oscillations at at least one, dominating, wanted oscillation frequency. As oscillation frequency for the oscillations in the wanted mode, in such case, usually an instantaneous natural eigen, or resonance, frequency of the tube arrangement is selected, which frequency, in turn, is essentially dependent on the size, shape and material of the measuring tubes as well as on an instantaneous density of the medium. As a result of the fluctuating density of the medium to be measured, and/or as a result of performing a change of media during operation, the wanted oscillation frequency is variable during operation of the measuring transducer naturally at least within a calibrated and, insofar, predetermined, wanted frequency band, which correspondingly has a predetermined lower and a predetermined upper limit frequency.
For defining a free oscillatory length of the measuring tubes, and associated therewith, for adjusting the wanted frequency band, measuring transducers of the above described type comprise additionally most often at least one inlet-side coupling element for forming inlet-side oscillation nodes for opposite phase vibrations, especially bending oscillations, of both measuring tubes, which element is affixed to both measuring tubes spaced apart from both flow dividers, as well as at least one outlet-side coupling element for the forming of outlet-side oscillation nodes for opposite phase vibrations, especially bending oscillations of the measuring tubes, which element is affixed to both measuring tubes, spaced apart from both flow dividers as well as from the inlet-side coupling element. In the case of straight measuring tubes, a minimum distance between inlet side and outlet side coupling elements—insofar as they belong to the tube arrangement—corresponds to, in such case, the free oscillatory length of the measuring tubes. By means of the coupling elements, additionally an oscillation quality factor of the tube arrangement, such as the sensitivity of the measuring transducer, can also be, on the whole, influenced in such a manner that, for a minimum required sensitivity of the measuring transducer, at least one minimum free oscillatory length is to be provided.
Development in the field of measuring transducers of vibration-type in the meantime has reached a state such that modern measuring transducers of the described type can, for practical purposes, satisfy highest requirements with respect to precision and reproducibility of the measurement results for a broad application spectrum in the field of flow measurement technology. As a result, such measuring transducers are used in practice for applications with mass flow rates from only a few g/h (grams per hour) up to some t/min (tons per minute), at pressures of up to 100 bar for liquids or even over 300 bar for gases. Due to the high bandwidth of their opportunities for use, industrial grade measuring transducers of vibration-type are available with nominal diameters (corresponding to the caliber of the pipeline to be connected to the measuring transducer, or to the caliber of the measuring transducer measured at the connecting flange), which lie in a nominal diameter range between 1 mm and 250 mm, and are specified for maximum nominal mass flow rate 2200 t/h, respectively, for pressure losses of less than 1 bar. A caliber of the measuring tubes lies, in such case, for instance, in a region between 80 mm and 100 mm.
As already mentioned, with measuring systems having measuring tubes executing bending oscillations, the viscosity, or also measured variables dependent upon it, such as, for instance, the Reynolds number, can also be ascertained, measurable based on the viscosity, and, indeed, also with bending oscillations (see also US-A 2006/0150750) However, in the case of this method, particularly also as a result of the often very small amplitude of the wanted oscillations, the sensitivity of the measuring transducer can have a certain dependency on the nominal diameter, and, indeed, in such a manner that the sensitivity decreases with the increasing nominal diameter. As a result, also the accuracy of measurement can become less with the increasing nominal diameter, or the respective transmitter electronics is presented with increased requirements with regard to signal processing technology and computing power. In spite of this, in the meantime, measuring transducers are also available for the purposes of measuring viscosity for use in pipelines with very high mass flow rates, and associated therewith, very large calibers of over 50 mm; there is quite a significant interest in measuring transducers of high precision and low pressure loss also for viscosity measurements in the case of still greater pipeline calibers, for instance, 100 mm or more, or mass flow rates of 1200 t/h or more, to be used, for instance, for applications in the petrochemical industry, or in the area of transporting and handling petroleum, natural gas, fuels, etc. This leads, in the case of a correspondingly scaled enlargement of already established measuring transducer concepts known from the state of the art, especially as set forth in EP-A 1 001 254, EP-A 553 939, U.S. Pat. No. 4,793,191, US-A 2002/0157479, US-A 2007/0151368, U.S. Pat. No. 5,370,002, U.S. Pat. No. 5,796,011, U.S. Pat. No. 6,308,580, U.S. Pat. No. 6,711,958, U.S. Pat. No. 7,134,347, U.S. Pat. No. 7,350,421, or WO-A 03/027616, to the fact that the geometric dimensions—especially the installed length corresponding to a distance between the sealing surfaces of both flanges, and in the case of curved measuring tubes, to a maximum lateral expansion of the measuring transducer—especially as resulting from the desired oscillation characteristics, the required loading capacity, as well as the maximum allowed pressure loss, would become very large. Associated therewith, also the empty mass of the measuring transducer unavoidably increases, with conventional measuring transducers of large nominal diameters already implemented having an empty mass of, for instance, 400 kg. For measuring transducers with two bent measuring tubes, for instance, according to U.S. Pat. No. 7,350,421 or U.S. Pat. No. 5,796,011, investigations have been performed concerning their scaling to still greater nominal diameters. These investigations have shown, for example, that for nominal diameters of more than 300 mm, the empty mass of a conventional measuring transducer enlarged to scale would lie well over 500 kg, along with an installed length of more than 3000 mm and a maximum lateral expansion of more than 1000 mm. As a result, it can be understood that industrial grade, even series-manufacturable, measuring transducers of conventional design and materials with nominal diameters of well over 300 mm will, both for reasons of technical feasibility and due to economic considerations, not be available in the foreseeable future.